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  <div class="question_difficulty">
   难度：Hard
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  <div>
   <h1 class="question_title">
    301. Remove Invalid Parentheses
   </h1>
   <p>
    Remove the minimum number of invalid parentheses in order to make the input string valid. Return all possible results.
   </p>
   <p>
    <strong>
     Note:
    </strong>
    &nbsp;The input string may contain letters other than the parentheses
    <code>
     (
    </code>
    and
    <code>
     )
    </code>
    .
   </p>
   <p>
    <b>
     Example 1:
    </b>
   </p>
   <pre>
<b>Input:</b> "()())()"
<b>Output:</b> ["()()()", "(())()"]
</pre>
   <p>
    <b>
     Example 2:
    </b>
   </p>
   <pre>
<b>Input:</b> "(a)())()"
<b>Output:</b> ["(a)()()", "(a())()"]
</pre>
   <p>
    <b>
     Example 3:
    </b>
   </p>
   <pre>
<b>Input:</b> ")("
<b>Output: </b>[""]
</pre>
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   <h1 class="question_title">
    301. 删除无效的括号
   </h1>
   <p>
    删除最小数量的无效括号，使得输入的字符串有效，返回所有可能的结果。
   </p>
   <p>
    <strong>
     说明:
    </strong>
    输入可能包含了除&nbsp;
    <code>
     (
    </code>
    &nbsp;和&nbsp;
    <code>
     )
    </code>
    &nbsp;以外的字符。
   </p>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <pre><strong>输入:</strong> "()())()"
<strong>输出:</strong> ["()()()", "(())()"]
</pre>
   <p>
    <strong>
     示例 2:
    </strong>
   </p>
   <pre><strong>输入:</strong> "(a)())()"
<strong>输出:</strong> ["(a)()()", "(a())()"]
</pre>
   <p>
    <strong>
     示例 3:
    </strong>
   </p>
   <pre><strong>输入:</strong> ")("
<strong>输出: </strong>[""]</pre>
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